1. Field of the Invention
This invention relates, generally, to analyses using devices for liquid and gas chromatography, and particularly to very small chromatographic devices and analyses.
2. Prior Art
Chromatography is the group of separation techniques in which a mobile phase (either a gas or liquid) is flowed over a stationary phase (either liquid or solid). As the mobile phase moves past the stationary phase, repeated adsorption and desorption, or partitioning, of the solute occurs at the rate determined chiefly by the solute's ratio of distribution between the two phases (partition ratio, K).
A gas chromatograph (GC) is an analytical instrument which uses the principle discussed above to separate and identify the solute compounds as a gas that are present in a sample. Typical GCs will include an analytical column with a gas carrier source and sample inlet at one end and a detector at the other end. The device which obtains the sample, and which extracts the analytes from the sample, can either be integrated with the GC or be a separate device.
Most GC's will include a means of heating the column as the analytes are moved through the analytical column by the carrier gas. In conventional GC's the heating is achieved by enclosing the entire column within an oven, and the oven GC's are very bulky devices. Some advances have been made in the field to allow for smaller gas chromatographs. For example, newer means for heating the column have been discovered. U.S. Pat. No. 5,611,846 to Overton et al discloses a GC which uses electrically resistive heater wire placed adjacent to the analytical column for heating of the column. This configuration allows for a device of less bulk than conventional oven GCs. However, even the device of the '846 patent is not built to such a miniature scale so as to be easily adaptable to personal sensor applications.
The need is clear and urgent for speciating chemical sensors that respond to the concentration of specific chemicals in complex mixtures. Conventional GCs and gas chromatograph/mass spectrometers (GCMS) have the capability to respond to the concentration of specific substances in complex mixtures but are certainly not "sensors" in terms of their size and functionality. If the speciating capability of certain conventional GC instruments could be fitted into the small size and operational functionality of common sensors, this development would result in a true speciated chemical sensor.
In this application, the name "GC sensor" will be used to describe this new type of analytical instrument that uses gas chromatographic technology for its speciating analytical capability. If GCs could be sufficiently miniaturized to function as GC sensors, multiple GC sensor modules could be placed in an array within a single instrument. Each GC sensor in this array could be designed for the performance of separate analytical functions. Redundant and simultaneous testing could be done, or each GC sensor could be designed to test for a specific compound. Currently, such selectivity is only achievable with large and expensive "hyphenated" analytical instruments such as GCMS.
Selectivity is extremely vital in GC in applications such as bomb and chemical warfare agent detection. Multiple GC sensor modules can also be tailored to provide analyses of compounds with widely different chemical characteristics. For example, one sensor module could be fitted with a molecular sieve column for separation of the permanent gases such as hydrogen, nitrogen and oxygen while another sensor module could be fitted with a column for the separation of BTEX components, or hydrocarbons in the range C14 to C25.
In many GC applications one desires to perform testing which requires two or more GCs. If one wants to test across an extremely broad dynamic range one must use multiple instruments, with their attendant bulk. Miniature GC devices would allow such testing to be done in a field environment. Miniaturization also decrease the power requirements of conventional GCs.
Conventional GCs are currently designed to handle a very wide variety of analytical applications. The disadvantage of this design philosophy is that each instrument has much unused capability in any given application. Unused capability in analytical instruments translates into extra cost, size, power consumption, and complexity. There is a need for small, rugged, and relative inexpensive analyzers that have satisfactory performance within certain applications but, in general, cannot be applied to a wide variety of other applications. Analytical instruments using GC sensor modules need not have all the analytical capability of laboratory devices, but if needed the instruments could be designed with such capability.
Outside of the field of gas chromatography, many advances have been made in the miniaturization of mechanical as well as electrical devices. One technique for the manufacture of very small devices is the use of synchronous x-ray radiation, such as that available from a synchrotron, to irradiate material which is sensitive to the radiation. The material can then be etched, leaving very fine and intricate structures.
The remaining structure can itself be the actual desired structure, or it can be used as a mold for the electrodeposition of metal. Again, the deposited metal structure can be the desired structure, or it too can serve as a mold for other materials. The resulting device, whether created from a mold or directly from the etching process, is extremely small and has extremely high resolution. It also can have extremely tall, accurate, and sharp vertical structures, and for this characteristic the devices are referred to herein as high aspect ratio microstructures (HARMs). HARMs have been made in various configurations such as valves, switches and heat exchanger surfaces. However, the inventor is unaware of any adaptation of HARMs to gas chromatography.
Many applications of GC analysis, spanning a wide variety of fields, require decisions to be made based on the concentration of specific chemical compounds in complex mixtures. This type of analysis, analysis that provides data on concentrations of specific chemical species, is called compound-specific analysis. Compound specific analysis can be used in environmental, medical, industrial, transportation, energy, service/facilities, educational, military, and other applications.
Specific examples of applications for a device which incorporates one or more GC sensors capable of compound-specific analysis could include:
an airport security guard using the device to check for explosives; PA1 police using the device to search a cruise ship following a bomb treat; PA1 using the device to monitor a subway station for chemical nerve agents following reports of a strange odor in the station; PA1 emergency officials using several devices located throughout a city to detect chemical emissions during a fire at a nearby chemical manufacturing plant, and order evacuations as appropriate; PA1 a customs inspector using the device to detect contamination of foodstuffs by an illegal pesticide; PA1 a petrochemical plant increasing its efficiency and product quality by monitoring process streams with many devices to rapidly detect unwanted deviations from operational and product specifications; PA1 a medical professional using the device to perform rapid, inexpensive, non-invasive screening for metabolic diseases; and PA1 a commodity inspector using the device to detect the freshness of raw product.
Current "sniffing" technology is not technology at all, but instead relies on dogs in serious situations involving drugs and explosives. There are myriad other applications which will arise given our technology driven society.